Pscholo~- In The News UFO Sighting Over St. Louis ST. LOUIS, MO,]ANUARY

28,2000. A motorist has reported that two weeks ago she spotted three unidentified flying objects hovering over the highway during rush-hour traffic. Stacy McKenna, 28, a col­lege student and waitress, said the · objects were shaped like triangles with white lights at each point. "At first they were just two bouncing, glowing lights," she said. "Then another one dropped out of the sky. It was so huge, I screamed because I thought I was going to hit it." McKenna said she didn't believe in UFOs be­fore, but now she is "in­trigued."

Dozens of other area residents have reported seeing alien spacecraft this month. They are not alone: over the years, UFO sight­irigs have occurred throughout the world, es-

Are these alien spacecraft? Many people who viewed these odd objects in the skies above Santos, Brazil, were convinced they were seeing UFOs.

pecially in North Ameri­ca. California leads in the number ofreported sight­ings, with Washington State in second place. A private pilot helped coin the term "flying saucer" many decades ago after seeing nine shiny disks

skipping through the air, and thousands of reports of saucers and other mys­terious objects have come in since then.

One of the earliest UFO accounts occurred in the late 1940s, when a rancher noticed some

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SENSATION AND PERCEPTION

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W e have all heard stories about UFOs and "flying saucers." Some

of us scoff at them; others take them seriously. Why are these

accounts so common? Are UFO sightings reported only by

people who are silly or gullible, or do smart, savvy people also see alien

aircraft? If such sightings are illusions, then why are those who report them

so confident that what they saw was real? And why is one report often fol­

lowed by several others in the same location: because there really are a lot

of UFOs in the area or because of the power of suggestion?

In this chapter, we will try to answer these questions by exploring

how our sense organs take in information from the environment and how

the brain uses this information to construct a model of the world-a model

that is not always accurate. We will focus on two closely connected sets of

processes that enable us to know what is happening both inside our bod­

ies and in the world beyond our own skins.

The first process, sensation, is the detection of physical energy emit­

ted or reflected by physical objects. The cells that do the detecting are lo­

cated in the sense organs-the eyes, ears, tongue, nose, skin, and internal

body tissues. Sensory processes produce an immediate awareness of sound,

color, form, and other building blocks of consciousness. But to make sense

of the world conveyed to us through our senses, we also need perception,

a set of mental operations that organize sensory signals into meaningful

Our Sensational Senses

• Vision

• Hearing

• Other Senses

• Perceptual Powers:

Origins and Influences

• Puzzles of Perception

• Psychology In The News,

Revisited

• TAKING PSYCHOLOGY WITH YOU

Living with Pain

175

176 CHAPTER 6 Sensation and Perception

If you stare at the cube, the sur­face on the outside and front will suddenly be on the inside and back or vice versa, because your brain can interpret the sensory image in two different ways. The purple and white drawing below the cube can also be perceived in two ways. Do you see them?

sensation The detection of physical energy emitted or renect­ed by physica l objects

perception The process by which the brain organizes and Inter­prets sensory Information.

sense receptors Specialized cells that con­vert physical energy In the environment or the body to electrical energy that can be transmitted as nerve Impulses to the brain.

doctrine of specific nerve energies The principle that different sensory modalities exist because signals received by the sense organs stim­ulate different nerve path­ways leading to different areas of the brain.

patterns. Our sense of vision produces a two-dimensional image on the back of the eye, but we perceive the world in three di­mensions. Our sense of hearing brings us the sound of a C, an E, and a G played si­multaneously on the piano, but we perceive a C major chord.

Sensation and perception are the foun­dation of learning, thinking, and acting, and findings on these topics can often be put to practical use-for example, in the design of hearing aids and industrial robots, and in the training of flight controllers, as­tronauts, and other people who must make crucial decisions based on what they sense and perceive . An understanding of sensa­tion and perception can also help us think more critically about our own experiences. As you read this chapter, ask yourself why people sometimes perceive things that are not there and, conversely, why they some­times miss things that are there-looking without seeing, listening without hearing.

• What kind of "code" in the nervous system helps explain why a pinprick and a kiss feel dif ­

ferent?

• Why does your dog hear a "silent" doggie whis­tle when you can't?

• What kind of bias can influence whether you think you hear the phone ringing when you're in the shower?

• What happens when people are deprived of all external sensory stimulation?

Our Sensational Senses At some point, you probably learned that there are five senses, corresponding to five sense organs: vision (eyes), hearing (ears), taste (tongue), touch (skin), and smell (nose). Actually, there are more than five senses, though scientists disagree about the exact number. The skin, which is the organ of touch or pressure, also senses heat, cold, and pain, not to men­tion itching and tickling. The ear, which is the organ of hearing, also contains receptors that account for a sense of balance. The skeletal muscles contain re­ceptors responsible for a sense of bodily movement.

All of our senses evolved to help us survive. Even pain, which causes so much human misery, is an indispensable part of our evolutionary heritage, for it alerts us to illness and injury. But sensory ex­periences contribute immeasurably to our lives even when they are not helping us stay alive. They en­tertain us, amuse us, soothe us, inspire us. If we really pay attention to our senses, said poet William Wordsworth, we can "see into the life of things" and hear "the still, sad music of humanity."

The Riddle of Sepa rate Sensations Sensation begins with the sense receptors, cells lo­cated in the sense organs. When these receptors detect an appropriate stimulus-light. mechanical pressure, or chemical molecules-they convert the energy of the stimulus into electrical impulses that travel along nerves to the brain. Sense receptors are like military scouts who scan the terrain for signs of activity. These scouts cannot make many decisions on their own. They must transmit what they learn to field officers-sensory neurons in the peripheral nervous system (see Chapter 4). The field officers in turn must report to generals at a command center-the cells of the brain. The gen­erals are responsible for analyzing the reports, com­bining information brought in by different scouts, and deciding what it all means.

The "field officers" in the sensory system-the sensory nerves-all use exactly the same form of communication, a neural impulse . It is as if they must all send their messages on a bongo drum and can only go "boom." How, then, are we able to ex­perience so many clifferent kinds of sensations? The answer is that the nervous system encodes the mes­sages. One kind of code, which is anatomical, was first described in 1826 by the German physiologist Johannes Miiller, in his doctrine of specific nerve energies. According to this doctrine, different sen­sory modalities (such as vision and hearing) exist because signals received by the sense organs stim­ulate different nerve pathways leading to different areas of the brain. Signals from the eye cause im­pulses to travel along the optic nerve to the visual cortex. Signals from the ear cause impulses to trav­el along the auditory nerve to the auditory cortex. Light and sound waves produce different sensa­tions because of these anatomical differences.

The doctrine of specific nerve energies implies that what we know about the world ultimately reduces to what we know about the state of our own nervous system. Therefore, if sound waves

Sensation and Perception CHAPTER 6 177

could stimulate nerves that end in the visual part of the brain, we would "see" sound . In fact, a sim­ilar sort of crossover does occur in a rare condition called synesthesia, in which the stimulation of one sense also evokes a sensation in another. The person may say that the color purple smells like a rose, the aroma of cinnamon feels like velvet, or the sound of a note on a clarinet tastes like cher­ries (Cytowic, 2002; Martino & Marks, 2001). Synesthetes may have unusually direct connec­tions between different sensory areas of the brain, such as those that handle vision and hearing, or an unusual number of connections between these areas (Nunn et al., 2002).

Synesthesia, however, is the exception, not the rule; for most of us, the senses remain separate. Anatomical encoding alone does not completely explain why this is so, not does it explain varia­tions of experience within a particular sense-the sight of pink versus red, the sound of a piccolo ver­sus the sound of a tuba , or the feel of a pinprick versus the feel of a kiss. An additional kind of code is therefore necessary. This second kind of code has been called functional.

Functional codes rely on the fact that sensory receptors and neurons fire, or are inhibited from firing, only in the presence of specific sorts of stim­uli . At any particular time, then, some cells in the nervous system are firing, and some are not. In­formation about which cells are firing, how many cells are firing, the rate at which cells are firing, and the patterning of each cell's firing constitutes a func­tional code . You might think of such a code as the neurological equivalent of the Morse code, but much more complicated. Functional encoding may occur all along a sensory route, starting in the sense organs and ending in the brain.

Measuring the Senses Just how sensitive are our senses? The answer comes from the field of psychophysics, which is con­cerned with how the physical properties of stimuli are related to our psychological experience of them. Drawing on principles from both physics and psy­chology, psychophysicists have studied how the strength or intensity of a stimulus affects the strength of sensation in an observer.

Absolute Thresholds One way to find out how sensitive the senses are is to show people a series of signals that vary in intensity and ask them to say which signals they can detect. The smallest amount of energy that a person can detect reliably is known as the absolute threshold. The word absolute is a bit misleading, though, because people detect bor­derline signals on some occasions and miss them on others. "Reliable" detection is said to occur when a person can detect a signal 50 percent of the time.

If you were having your absolute threshold for brightness measured, you might be asked to sit in a dark room and look at a wall or screen. You would then be shown flashes of light, varying in brightness, one flash at a time. Your task would be to say whether you noticed a flash . Some flashes you would never see. Some you would always see. And sometimes you would miss seeing a flash, even though you had noticed one of equal bright­ness on other trials. Such errors seem to occur in part because of random firing of cells in the nervous system, which produces fluctuating background "noise", something like the background noise in a radio transmission.

By studying absolute thresholds, psychologists have found that our senses are very sharp indeed.

synesthesia A rare condition in which stimulation of one sense also evokes a sensation In another.

absolute threshold The smallest quantity of phySical energy that can be reliably detected by an observer.

Different species sense the VJorld differently. The flower on the left was photographed in normal light The one on the right, photographed under ultraviolet light, Is what a butterfly might see, because butterflies have ultra­violet receptors. The hundreds of tiny bright spots are nectar sources.

178 CHAPTER 6 Sensation and Perception

difference threshold The smallest difference In stimulation that can be reliably detected by an observer when two stimuli are compared; also called Just noticeable difference (jnd).

THE ELECTROMAGNETIC SPECTRUM Wavelength

3000 mi. 1 mi. 100 ft. 1 ft. .01 ft. .0001 ft. 10 nm. 1 nm. .001 nm. .00001 nm.

Gamma Cosmic. Radio Microwaves . Infrared · U-V X-rays rays · rays

.... .... .... .... .... .... .... .... ....-- .... ....Infrared Visible spectrum .... Ultraviolet ~------------~

1500 1000 700 600 500 400 300

Wavelength in nanometers

FIGURE 6.1 Visible Spectrum of Electromagnetic Energy Our visual system detects only a small fraction of the electromagnetic energy around us.

If you have normal sensory abilities, you can see a candle flame on a clear, dark night from 30 miles away. You can hear a ticking watch in a perfectly quiet room from 20 feet away. You can taste a tea­spoon of sugar diluted in 2 gallons of water, smell a drop of perfume diffused through a three-room apartment, and feel the wing of a bee falling on your cheek from a height of 1 centimeter (Galanter, 1962).

Yet despite these impressive sensory skills, our senses are tuned in to only a narrow band of phy­sical energies. For example, we are visually sensi­tive to only a tiny fraction of all electromagnetic energy; we do not see radio waves or microwaves (see Figure 6.1). Other species can pick up signals that we cannot. Dogs can detect high-frequency sound waves that are beyond our range, as you know if you have ever called your pooch with a "silent" doggie whistle. Bats and porpoises can hear sounds two octaves beyond our range, and bees can see ultraviolet light, which merely gives human beings a sunburn.

Difference Thresholds Psychologists also study sensory sensitivity by having people compare two stimuli and judge whether they are the same or different. For example, a person might be asked to

compare the weight of two blocks, the brightness of two lights, or the saltiness of two liquids. The small­est difference in stimulation that a person can de­tect reliably (again, half of the time) is called the difference threshold, or just noticeable difference

(jnd). When you compare two stimuli, A and B, the difference threshold will depend on the intensity or size of A. The larger or more intense A is, the greater the change must be before you can detect a difference. If you are comparing the weights of two pebbles, you might be able to detect a difference of only a fraction of an ounce, but you would not be able to detect such a subtle difference if you were comparing two massive boulders.

In everyday life, we may sometimes think we can detect a difference between stimuli when we cannot. Years ago, as a class project, undergraduate students at Williams College offered tasters three glasses of cola, two of one leading brand and one of the other (or vice versa), and asked them which drink they liked most and least. Each taster was given three trials. Most of the tasters were incon­sistent in their preferences, indicating that they had trouble telling the two brands apart (Solomon, 1979).

Signal-Detection Theory Despite their use­fulness, the procedures we have described have a serious limitation. Measurements for any given in­dividual may be affected by the person's general tendency, when uncertain, to respond, "Yes, I noticed a signal (or a difference)" or, "No, I didn't notice anything." Some people are habitual yea­sayers, willing to gamble that the signal was really there. Others are habitual naysayers, cautious and conservative. In addition, alertness, motives, and expectations can influence how a person responds

Sensation and Perception CHAPTER 6 179

on any given occasion . If you are in the shower and you are expecting an important call, you may think you heard the telephone ring when it clidn't. In laboratory studies, when observers want to im­press the experimenter, they may lean toward a positive response.

Fortunately, these problems of response bias are not insurmountable. Accorcling to signal-detection theory, an observer's response in a detection task can be divided into a sensory process, which depends on the intensity of the stimulus, and a decision process, which is influenced by the observer's re­sponse bias. Methods are available for separating these two components. For example, the researcher can include some trials in which no stimulus is pre­sent and others in which a weak stimulus is present. Under these conditions, four kinds of responses are possible: The person (1) detects a signal that was present (a "hit"), (2) says the signal was there when it was not (a "false alarm"), (3) fails to detect the sig­nal when it was present (a "miss"), or (4) correct­ly says the signal was absent when it was absent (a "correct rejection").

Yea-sayers will have more hits than naysayers, but they will also have more false alarms because they are too quick to say, "Yup, it was there ." Naysayers will have more correct rejections than yea-sayers, but they will also have more misses, because they are too quick to say, "Nope, nothing

was there." This information can be fed into a math­ematical formula that yields separate estimates of a person's response bias and sensory capacity. The individual's true sensitivity to a signal of any par­ticular intensity can then be predicted.

The old method of measuring thresholds as­sumed that a person's ability to detect a stimulus de­pended solely on the stimulus. Signal-detection theory assumes that there is no single "threshold, " because at any given moment, a person's sensitiv­ity to a stimulus depends on a decision that he or she actively makes. Signal-detection methods have many real-world applications, from screening ap­plicants for jobs requiring keen hearing to training air-traffic controllers, whose decisions about the presence or absence of a blip on a radar screen may mean the difference between life and death.

Sensory Ada ptation Variety, they say, is the spice of life. It is also the essence of sensation, for our senses are designed to respond to change and contrast in the environment. When a stimulus is unchanging or repetitious, sen­sation often fades or disappears. Receptors or nerve cells higher up in the sensory system get "tired" and fire less frequently. The resulting decline in sensory responsiveness is called sensory adapta­tion. Such adaptation is usually useful because it

LVED NOW YOU SEE IT, I~OW YOU DON'T \GET I Sensation depends on change and contrast in the environment. Hold your hand over one eye and stare at the dot in the middle of the circle on the right. You should have no trouble maintaining an image of the circle. However, if you do the same with the circle on the left, the)mage will fade . The gradual change from light to dark does not provide enough contrast to keep your visual receptors firing at a steady rate. The circle reappears only if you close and reopen your eye or shift your gaze to the x .

•

signal-detection theory A psychophysical theory that divides the detection of a sensory signal into a sensory process and a decision process.

sensory adaptation The reduction or disap­pearance of sensory re­sponsiveness that occurs when stimulation Is un­changing or repetitious.

---- - --

180 CHAPTER 6 Sensation and Perception

sensory deprivation spares us from having to respond to unimportant in­The absence of normal formation; for example, most of the time you have levels of sensory no need to feel your watch sitting on your wrist. stimulation. Sometimes, however, adaptation can be hazardous,

as when you no longer smell a gas leak that you noticed when you first entered the kitchen.

We never completely adapt to extremely in­tense stimuli-a terrible toothache, the odor of am­monia, the heat of the desert sun. And we rarely adapt completely to visual stimuli, whether they are weak or intense. Eye movements, voluntary and involuntary, cause the position of an object's image on the back of the eye to keep changing, so that visual receptors located there don't have a chance to "fatigue." But in the laboratory, re­searchers can stabilize the image of a simple pat­tern, such as a line, at a particular point on the back of a person's eye . They use an ingenious device

The effects of sensory deprivation depend on the cir­consisting of a tiny projector mounted on a con­

cumstances. Being isolated against your will can be tact lens. Although the eyeball moves, the image terrifying, but many people have found an hour alone of the object stays focused on the same receptors. in a "flotation tank" to be pleasantly relaxing. In minutes, the image begins to disappear.

What would happen if our senses adapted to most incoming stimuli? Would we sense nothing, or orienting than was thought at first. In fact, many would the brain substitute its own images for the people enjQY time-limited per~privation, sensory experiences no longer available by way of and some perceptual and intellectual abilities actu­the sense organs? In early studies of sensory de- ally improve. The response to sensory deprivation

---RriY$ltion, researchers studied this question byi~ depends on your expectations and interpretations of lating male volunteers from all patterned sight and what is happening. Reduced sensation can be scary sound. Vision was restricted by a translucent visor, if you are locked in a room for an indefinite period, hearing by a U-shaped pillow and by background but relaxing if you have retreated to that. room vol­noise from an air conditioner and fan, and touch by untarily for a little time out-at, say, a lUXury spa or cotton gloves and cardboard cuffs. The volunteers a monastery. took brief breaks to eat and use the bathroom, but Still, it is clear that the human brain requires otherwise they lay in bed, doing nothing. The re­ a minimum amount of sensory stimulation in order sults were dramatic. Within a few hours, many of the to function normally. This need may help explain r;o~n felt edgy. Some w ere so OiS<5rtented-t-ha-t-they why people who live alone often keep the radio or quit-rhesW<fY-ihdlrstda¥. Th(;sewh~ s-ta,yed-j0nier television set running continuously and why pro­beEame confused, restless,al1dgrouchy. Many re­ longed solitary confinement is used as a form of ported bizarre visions, such as a squadron of march­ punishment or even torture. ing squirrels or a procession of marching eyeglasses. ,Few were. willing to remain in the-study for more thal!Jwo or three days (Heron, 1957). Sensory Overload

, / But the notion that sensory deprivation is un- If too little stimulation can be bad for you, so can - pleasant or even dangerous turned out to be an too much, because it can lead to fatigue and men-

oversimplification (Suedfela;T975}-: " tal confusion. If you have ever felt exhausted, ner-Thinking Critically In many of the studies, the experi- vous, and headachy after a day crammed with About Sensory mental procedures themselves prob- hectic activities and deadlines, you know firsthand Deprivation

ably aroused anxiety: Participants about sensory overload. wernoTd" ~D6ul"'p'crnic15liftons" aiid - When people find themselves in a state of over-

were asked to sign "release from legai liability" load, they often cope by blocking out unimportant forms. Later research, using better methods, showed-· sights and sounds and focusing only on those they that hallucinations are less dramatic and less dis- find interesting or useful. Psychologists have dubbed

Sensation and Perception CHAPTER 6 181

If you are not overloaded, try answering these questions.

1. Even on the clearest night, some stars cannot be seen by the naked eye because they are below the viewer's threshold.

2. If you jump into a cold lake but moments later the water no longer seems so cold, sensory ____ has occurred.

3. If you are immobilized in a hospital bed, with no one to talk to and no TV or radio, and you feel edgy and disoriented, you may be suffering the effects of ____

4. During a break from your job as a server in a restaurant, you decide to read. For 20 minutes, you are so engrossed that you fail to notice the clattering of dishes or orders being called out to the cook. This is an example of _____

5. In real-life detection tasks, is it better to be a naysayer or a yea-sayer?

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this the "cocktail party phenomenon" because at a cocktail party, a person typically focuses on just one conversation, ignoring other voices, the clink of ice cubes, music, and bursts of laughter across the room. The competing sounds all enter the nervous system, enabling the person to pick up anything important-such as the person's own name, spoken by someone several yards away. Unimportant sounds, though, are not fully processed by the brain.

The capacity for selective attention protects us in daily life from being overwhelmed by all the sensory signals bombarding our receptors. The brain is not forced to respond to everything the sense receptors send its way. The "generals" in the brain can choose which "field officers" get past the command center's gates. Those that do not seem to have anything important to say are turned back.

• How does the eye differ from a camera?

• Why can we describe a color as bluish green

but not as reddish green?

• If you were blind in one eye, why might you misjudge the distance of a painting on the wall

but not of buildings a block away?

• As a friend approaches, her image on your reti­na grows larger; why do you continue to see

her as the same size?

• Why are perceptual illusions valuable to psy­

chologists?

Vision is the most frequently studied of all the senses, and with good reason. More information about the external world comes to us through our eyes than through any other sense organ. (Perhaps that is why people say "1 see what you mean" instead of "1 hear what you mean.") Because we are most active in the daytime, we have been "wired" by evolution to take advantage of the sun's illumination. Animals that are actjve at night tend to rely more heavily on hearing.

What We See The stimulus for vision is light; even cats, raccoons, and other creatures famous for their ability to get

selective attention The focusing of attention on selected aspects of the environment and the blocking out of others.